In the preceding years of this project we have developed a unique instrument to assess noninvasively the concentration and morphology of transfusion platelets via a rapid, clinically acceptable procedure. The basis of this measurement is low-angle multiple scattering of light and the changes in this signal associated with orientation of the subset of the platelet population that is discoid. The instrument, under microprocessor control, calculates the concentration and percentage of platelets that are discoid within a transfusion bag. This requires fewer than three minutes. These measurements are being used in a variety of research studies of the effects of various storage conditions (e.g., temperature transients, bag wall material, addition of pharmacological agents) on the condition of stored platelet concentrates. The instrument was originally designed to play a role in improving the quality of platelet transfusions, by routine and accurate noninvasive assessment of platelet quality (as represented by the degree of normal platelet morphology). To this purpose ongoing studies at the Univ. of VA Medical School Blood Bank and National Labs of the American Red Cross are directed at determining the degree of correlation between the noninvasive measurement and clinical efficacy of the platelet transfusion.